When I try to change the storage to External, it wants to reformat my drive.
Is there a way to get around this? I’ve already got a drive loaded up with movies.
Drive is formatted to HFS+
Yes, you do not need to reformat (assuming your external drive was previously synced to Apple TV). If you get a USB stick you can perform the operation with that. i.e. Switch to external (to the USB Stick).
This may vary from version to version but in my process of doing this the test USB stick I used was renamed MediaHD
Once the format of the USB stick was complete I plugged my current external drive that contained all my movies directly to my mac and renamed it on the desktop to match precisely the USB stick name that ATV Flash gave it. (in this case MediaHD).
WARNING: Do not change any of the existing files or folders on the main drive as you may upset the iTunes sync of your original drive.
Your main drive should now sync as normal when you plug it back to the ATV and reboot.